1. Field of the Invention
The invention relates to the process of fabricating integrated circuits. More specifically, the invention relates to a method and an apparatus for fracturing polygons used in a lithography process for fabricating integrated circuits.
2. Related Art
Recent advances in integrated circuit technology have largely been accomplished by decreasing the feature size of circuit elements on a semiconductor chip. As the feature size of these circuit elements continues to decrease, circuit designers are forced to deal with problems that arise as a consequence of the optical lithography process that is typically used to manufacture integrated circuits. This optical lithography process begins with the formation of a photoresist layer on the surface of a semiconductor wafer. A mask composed of opaque regions, which are generally formed of chrome, and light-transmissive clear regions, which are generally formed of quartz, is then positioned over this photoresist layer. (Note that the term “mask” as used in this specification is meant to include the term “reticle.”) Light is then shone on the mask from a visible light source, an ultraviolet light source, or more generally some type of electromagnetic radiation source together with suitably adapted masks and lithography equipment.
This image is reduced and focused through an optical system containing a number of lenses, filters, and mirrors. The light passes through the clear regions of the mask and exposes the underlying photoresist layer. At the same time, opaque regions of the mask block the light leaving underlying portions of the photoresist layer unexposed.
The exposed photoresist layer is then developed, through chemical removal of either the exposed or non-exposed regions of the photoresist layer. The end result is a semiconductor wafer with a photoresist layer having a desired pattern. This pattern can then be used for etching underlying regions of the wafer.
The masks used to expose the photoresist layer are typically processed by an optical proximity correction (OPC) process to alleviate problems cause by the diffraction of the radiation at the feature edges, and the over-etching of the photoresist at the ends of the features. After the OPC process, the mask features are often fractured so that each exposure element is a rectangle or a trapezoid. This fracturing process can lead to problems when a feature on the mask includes a notch near a hole.
For example, FIG. 1A illustrates a mask feature 102 with a notch 107 near a hole 104. Note that feature 102 includes “hole route” 106, which allows a vertex trace to enter and exit the hole 104 in feature 102. A polygon can be represented by a vertex trace, which traverses the polygon edges in a counterclockwise fashion. In FIG. 1A, for example, a vertex trace can start at the upper right vertex labeled 1 and proceed counterclockwise to the upper left vertex labeled 2 then to lower left vertex labeled 3 and so on. The trace terminates when it reaches the point preceding the initial point. Note that while a counterclockwise vertex trace is described, a clockwise vertex trace would work as well.
When the trace enters a hole, the hole is traced in a clockwise fashion. Note that this is consistent with the counterclockwise trace on the outside of the polygon because it keeps the polygon interior to the left of the trace. In FIG. 1A, the entrance to the hole is from the vertex at the right end of hole route 106 (vertex 7). The next vertex, vertex 8, is at the upper right of the hole. Note that the exit vertex, vertex 12, is at the same point as vertex 7.
During the mask writing process, in order to simplify the polygon, it is desirable to eliminate such holes. This is typically accomplished by making a cut on the polygon. For example, FIG. 1B illustrates how cut 108 is added to feature 102 to divide feature 102 into an upper portion 110 and a lower portion 112 as is illustrated in FIG. 1C—drawn exploded apart for clarity. The fracturing process then continues to make additional cuts to fracture the polygons for the upper portion 110 and lower portion 112 into rectangles and trapezoids.
FIG. 1D illustrates how a sliver 116 is generated in the lower portion 112 of feature 102. If the fracturing process places cut 114 at the vertex of notch 107 as shown, sliver 116 is created. Note that the width of sliver 116 may be smaller than a minimum sliver width, which may cause problems during subsequent mask writing and fabrication operations.
A sliver is a rectangle or a trapezoid whose minimum width is below a user-defined threshold. The polygon-fracturing algorithm targets “shaped beam” electron photolithography, where the mask is exposed by photographic equipment, which directs the beam through a keyhole aperture. If this aperture is too narrow, the exposure will be less successful than normal. Machine parameters dictate what this minimum aperture width is; any shape which falls below this minimum aperture width is termed a “sliver.”
FIG. 1E illustrates another possibility for fracturing the lower portion 112 of mask feature 102. In an alternate attempt to fracture the polygon into rectangles, the fracturing process places cut 118 at the vertex of notch 107 as shown, creating sliver 120. The width of sliver 120 may also be smaller than a minimum sliver width, and may also cause problems during subsequent mask writing and fabrication operations. Note that while the above discussion relates to photo mask writing systems, the same problems of creating slivers can exist in direct write lithography systems also.
Hence, what is needed is a method and an apparatus for fracturing polygons on a mask used in an optical lithographic process without the problems described above.